Field of the Invention
Embodiments described herein relate generally to an optical system adjustment method of an image acquisition apparatus. For example, embodiments described herein relate generally to an image acquisition apparatus for acquiring a secondary electron image of a pattern emitted due to irradiation with multiple beams using electron beams.
Related Art
In recent years, with increasingly higher integration and larger capacities of large scale integrated circuits (LSI), circuit linewidths required for semiconductor devices become increasingly narrower. These semiconductor devices are manufactured by using an original image pattern (also referred to as a mask or a reticle. Hereinafter, referred to generically as a “mask”) on which a circuit pattern is formed, and exposing and transferring the pattern onto a wafer using a reduction projection exposure apparatus called a stepper to form a circuit.
Improvements in yield are indispensable for the manufacture of LSI that needs a large amount of manufacturing costs. However, as representatively shown by 1 GB DRAM (random access memory), patterns constituting LSI have transitioned from sub micrometer to that of nanometer. In recent years, with increasingly finer dimensions of LSI patterns formed on semiconductor wafers, dimensions that need to be detected as pattern defects are now extremely small. Therefore, in an image pickup apparatus represented by a pattern inspection apparatus for inspecting defects of an ultrafine pattern transferred onto a semiconductor wafer, the acquisition of a highly accurate pattern image is required.
For example, in an inspection apparatus as an example of an apparatus that captures a pattern image, a pattern formed on a substrate such as a semiconductor wafer or a lithography mask by using an enlarging optical system is captured at a predetermined magnification using a laser light to obtain an optical image. Then, a method of conducting inspection by comparing such an optical image with design data or an optical image obtained by capturing the same pattern on a target object is known. For example, “die to die inspection” that compares optical image data capturing the same pattern in different places on the same mask and “die to database inspection” that, when writing pattern-designed CAD data to a mask as a pattern, inputs pattern writing data (design pattern data) converted into an apparatus input format for a lithography apparatus to input into an inspection apparatus, generates design image data (reference image) based thereon, and compares this data with an optical image as measurement data capturing the pattern are available as the pattern inspection method. In an inspection method in such an inspection apparatus, an inspection target substrate is placed on a stage and the stage is moved for a luminous flux to scan the target object to conduct inspection. The inspection target substrate is irradiated with a luminous flux by a light source and an illuminating optical system. Light transmitted or reflected by the inspection target substrate is formed as an image on a sensor via an optical system. An image captured by the sensor is sent to a comparison circuit as measurement data. After images are aligned, the comparison circuit compares the measurement data and reference data using an appropriate algorithm and if not matched, determines that there are pattern defects.
In the above pattern inspection apparatus, an inspection target substrate is irradiated with laser light and a transmitted image or a reflected image thereof is captured to acquire an optical image. On the other hand, development of an inspection apparatus that irradiates a target substrate with an electron beam, detects secondary electrons corresponding to each beam emitted from the target substrate, and acquires a pattern image has been advanced. With regard to an apparatus that captures a pattern using an electron beam represented by an inspection apparatus, development of a method using multiple beams is advanced, including a method using a single beam (see, for example, WO 2010/1037257 A).
In an apparatus for capturing a pattern using multiple beams, an adjustment of a primary optical system that irradiates a target object surface with multiple beams to be a primary beam and an adjustment of a secondary optical system that guides multiple secondary electrons emitted from the target object surface to respective desired positions detected individually by a multi-detector are needed. However, when multiple beams are used, if the adjustment of the secondary optical system is not completed, a secondary electron image cannot be obtained by the multi-detector, so the adjustment of the primary optical system using the secondary electron image cannot be made. Conversely, if the adjustment of the primary optical system is not completed, the secondary electrons deviate from an optical path of the secondary optical system, and the adjustment of the secondary optical system cannot be made. Thus, a contradiction arises such that if the adjustment of the secondary optical system is not completed, the primary optical system cannot be adjusted. Conversely, if the adjustment of the primary optical system is not completed, the secondary optical system cannot be adjusted.